Negative pressure wound therapy (“NPWT”) is an effective technology for treating open wounds. NPWT devices were originally accepted by the US Food and Drug Administration (“FDA”) in 1995, when the FDA approved a 510(K) for the Kinetic Concepts Inc. (“KCI”)'s V.A.C.® device. The definition of NPWT devices by the FDA has changed over the years; in general terms, its definition is: a system that is used to apply negative pressure for wound management purposes, including the removal of fluids (i.e., wound exudates, irrigation fluids, and infectious materials). The negative pressure is applied through a porous dressing positioned into or over the wound cavity, depending on wound type and depth, or over a flap or graft; the dressing distributes the pressure while removing fluids from the wound. NWPT systems typically include:                Non-adhesive wound dressing used to fill the wound cavity (e.g., a sterilized medical sponge or gauze; a.k.a., non-adhesive packing materials);        Drainage tube placed adjacent to or into the dressing;        Occlusive transparent film placed over the dressing (and potentially the drainage tube) and adhered to the skin to maintain a seal;        Collection container for drained fluids from the wound; and        Low pressure vacuum source.        
NPWT has been approved by the FDA to treat many wound types: chronic, acute, traumatic, sub-acute and dehisced wounds, partial-thickness burns, ulcers (such as diabetic, venous or pressure), surgically closed incisions (a.k.a., closed surgical incisions), flaps and grafts. The prescribed therapy time depends on wound type, wound dimensions, and patient conditions; it typically lasts from four weeks to four months. Disposable dressing components are changed approximately every three days.
Extensive clinical trials have demonstrated the success of negative pressure in healing the approved wound types by applying a controlled negative pressure typically between 20 mmHg and 200 mmHg. Most studies applied a constant vacuum pressure, with 125 mmHg being the most common, although cyclic and intermittent studies are currently underway. Evidence supporting the use of NPWT in the treatment of chronic, non-healing wounds exists primarily in the form of nonrandomized, controlled trials; prospective and retrospective large and small case series; single-center studies; and single case studies, with few randomized, controlled clinical trials. Studies also exist that demonstrate NPWT benefits in healing acute wounds. Additionally, since 2006, benefits of managing surgical incisions post-operatively have been shown with improved clinical outcomes; at least ten studies have been published to date. From these studies, proven medical benefits of NPWT treatment include:                Promotes blood flow (perfusion) at the wound;        Removes interstitial fluid (a.k.a., wound exudates), reduces edema;        Decreases counts of bacteria and infectious materials;        Increases rate of granulation tissue formation, reducing scar tissue formation, increases growth factors and fibroblasts;        Uniformly draws the wound edges together;        Provides a protected healing environment; and        Provides a moist environment.        
Although significant clinical evidence exists to support the benefit of NPWT as a safe therapy in healing chronic wounds, it is possible during NPWT to rupture a vein or artery. Usually, a machine safety alarm will signify a fluid leak rate that exceeds the rate that the machine was designed for. This alarmed leak rate typically includes the combination of both air and liquid, and typically has an upper safety limit of the minimum blood flow rate possible out of a wound cavity with an actively bleeding vein or artery. If a vein or artery accidently ruptures, the system must shut down. Therefore, it is very important to have a safety feature that stops blood flow if this occurs, in order not to exsanguinate the patient.
Lina et al. describe in U.S. Pat. No. 7,611,500 and WO1996/005873 an initial apparatus used for NPWT. In practice, the device proved to be effective; however, one major limitation was detected: the high electrical grid power source needed to operate the device limited the mobility of a patient. Therefore, future refinements, such as that described by Hunt et al. in U.S. Pat. No. 6,142,982, incorporated rechargeable batteries for the power source. Batteries increased patient mobility, but time was limited by the life of the batteries between charges. Additionally, battery management became an issue, especially for facilities with a high number of NPWT patients, and electrical grid power was still needed to recharge the batteries.
Eliminating the need for electrical power, via the grid or batteries, would create a more widely applicable, clinically viable therapy. The power requirement variability of a system is dependent on the desired vacuum pressure, rate of wound exudate removal from the wound cavity, and the leak rate of air into the system. As the air leak rate increases, more power is needed to supply a continuous negative pressure at a predetermined value or threshold range at the wound bed. Air leakage into the NPWT system requires the most power of any other component. Air leaks are the obstacle to creating a vacuum system that does not require a continuous external power source or frequent recharging of its internal power storage. Therefore, the feasibility of a mechanical NPWT system is heavily reliant on the seal quality of every interface in the system. The dressing system has been identified as the main source of air leaks in current NPWT systems, particularly at the interfaces between 1) the dressing and the skin and 2) the tube and the dressing. The amount of air leaks into these interfaces determines the time frequency that the pump needs to be recharged and the magnitude of vacuum pressure applied to the wound cavity at a specific time. These two latter characteristics are dependent system parameters.
Few mechanical NPWT systems are currently available, as described by the present inventor in “Development of a simplified Negative Pressure Wound Device” submitted in 2007 for her Master of Science in Mechanical Engineering at the Massachusetts Institute of Technology. Certain lower-pressure, mechanical devices were disclosed later by Hu et al. in U.S. Patent Application No. 2010/0228205. Current mechanical systems typically use sophisticated-material, planar dressings, such as hydrocolloid dressings, to try to solve the air leak problem. However, the inherent geometry mismatch of a planar dressing and the contoured skin surface often leads to air leaks. The mechanical devices therefore are only applicable for select, relatively flat surfaces on the body and, even then, it is difficult to eliminate air leaks entirely.
Non-electrical pumps are at the low end of the spectrum of medical pumps, typically utilizing bladder pumps and capillary action materials. Bladder pumps are used for both extracting and inserting fluids. By their physical characteristics, they are governed by non-linear spring like properties. Currently, bladder pumps are used in wound treatments for drainage purposes, particularly for internal, body cavity drainage. C. R. Bard, Inc. manufacturers many of these non-electrical pumps; one bladder model frequently used to drain internal cavities is commonly referred to as a Jackson Pratt Drain.
There are various limitations to applying NPWT with existing mechanical, bladder pumps. There are no pressure gauges on the pumps and, therefore, the user does not know the initial magnitude of the negative pressure pulled, and cannot monitor the pressure during therapy. Additionally, there are no air leak detection systems for the current pumps, except to visually watch for the expansion of the bladder at a rate higher than expected. If the pump is clear, one can also visually monitor if the expansion rate is due to air leaks or due to drainage fluid.
Capillary action materials are also currently used to treat wounds by providing very low negative pressure treatment, too low to be considered NPWT. This form of treatment is usually found in dressings such as small topical bandages to provide NPWT-like benefits to very small, self-healing wounds, such as blisters and brush burns. Treating a wound with this technology enhances the healing environment. Capillary action materials are filled with small capillaries between the wound and outside environment. A negative pressure is applied by capillary action of fluid flowing from the wound to the outside environment, thereby, removing interstitial fluid. One example of a capillary action material is Johnson & Johnson's First Aid Advanced Care Advanced Healing Adhesive Pads.
Dressing technologies have tried to address the issue of air leaks into NPWT systems. This is important to both electrical and mechanical systems to reduce their necessary power requirements. In mechanical systems, it is necessary for clinically relevant device functionality, such that power input and pump recharge time is reasonable for a caregiver and/or patient to perform. For electrical systems, air leak reduction reduces the number of, if not completely eliminates, false-positive, alarmed emergency system shutdowns. Air leak reduction allows battery designs to last longer on one battery charge and use lower power capacity batteries altogether. Air leak elimination potentially eliminates the need for a continuous power supply, as the vacuum pressure can be maintained in the occlusive environment within a specified threshold, for which the timeframe depends on the pump parameters and exudate removal rate (typically less than 100 mL/day) from the wound.
Currently, most NPWT dressings (the drape component) are thin, planar, tape-like adhesive dressings that must be applied to a contoured area of skin. A backing on the dressing must be removed to expose the adhesive, and then the dressing is applied to the skin. The pre-application handling of the dressings alone introduces a probability for air leaks, as the dressing typically folds onto itself or creases very easily due to its low bending stiffness; many dressings are thinner than a piece of standard paper, and the bending stiffness of a material is proportional to the inverse of its thickness cubed. As a dressing is applied, it must often fold onto itself in order to accommodate for a geometrical mismatch between the planar dressing and the contours of the body surrounding the wound to be treated. This creates creases, also referred to herein as wrinkles, in the dressing that have a high potential for causing air leaks into the NPWT system.
Adding to the geometrical mismatch, the dressings often become less adhesive due to the introduction of foreign materials onto the adhesive before dressing application. This is most common and almost unavoidable at the edges of the dressing due to handling by the caregiver. At times, the caregiver's hands introduce enough foreign particles onto the adhesive to forbid further adhesion of that area of the dressing. In the U.S., this often happens when a caregiver uses powdered gloves. This is a critical issue as the edges of the dressing are an area where leak propagation from the edge of the dressing to the wound cavity is potentially very high, based on the theory of interface fracture mechanics.
For the electrical NPWT systems, a thin plastic, adhesive backed dressing is typically used. Electrical NPWT dressing systems have not readily addressed the air leak issues listed above that form at the dressing-to-skin interface. Instead, dressing iterations have focused on air leaks at the tube-to-dressing interface. When NPWT was first introduced into the market, the drainage tube was inserted into the wound cavity through the edge of the dressing. This introduced a high potential for air leaks, which often alarmed the shut-off system. Caregivers began to solve this problem by raising the tube from the skin surface at the dressing edge, and pinching the dressing under the tube before the dressing contacts the skin. This caused the dressing to adhere to itself in an upside-down “T” pattern onto the skin.
Eventually, some of the NPWT dressing, commercial designs incorporated their own solutions to the high air leak rate at the tubing interface. Out of these solutions, the T.R.A.C. Pad by KCI was highly effective, which is driving the current design trends. The T.R.A.C. Pad prefabricates the drainage tube to a semi-rigid, tubing connector, which is then attached to a small, circular, planar adhesive dressing (a.k.a., drape). All of these connections are made air-tight during its manufacture. The tubing does not travel beyond the plane of the adhesive dressing, and therefore its opening remains at the skin surface. When the T.R.A.C. Pad is used, the standard dressing is initially applied to the wound, without a tubing connection. Then, a small incision is made in the dressing, over the wound cavity; this hole may also be prefabricated into the drape component of the dressing during its manufacture. The film backing of the circular adhesive component is removed from the Pad, and the tube opening is centered over the incision. Since the adhesive surface of the Pad is small, it is easier to handle than the procedure of tunneling the tube into the initial dressing. Although the Pad does not guarantee elimination of air leaks at the tube-to-dressing interface, it highly reduces the probable amount of air leaks into the dressing, based on its ergonomic design and small profile. A minimal amount of air leaks is almost unavoidable for all applications with planar adhesive components, due to the geometrical mismatch and user handling that still remain.
Many efforts have been made in order to overcome the identified barriers of low end, mechanical pumps for application in NPWT. Most of the focus has been on reducing air leaks and creating more predictable vacuum sources. New materials used in NPWT dressings have been the main driver in reducing the air leak rate into the system at the dressing-skin interface. These materials are often not new to wound dressings; however, they are new to NPWT. Pump design has been the focus of creating more predictable vacuum sources; mechanical components, such as linear or constant force springs, are often introduced into the system and maintain a more predictable behavior throughout therapy.
Only one mechanical NPWT system is on the market today, but is not widely used: SNaP® Wound Care System by Spiracur (Sunnyvale, Calif.). The SNaP® Wound Care System uses a hydrocolloid dressing with specific mechanical connectors from the tube to the dressing, in order to accommodate for air leaks; the provided hydrocolloid dressing is relatively small in size. Hydrocolloids are used in many wound-dressing systems, and are a common trend in the NPWT market. They are stiffer and thicker than most common, adhesive, planar, NPWT specific dressings. This causes the dressing to fold onto itself less during its handling and application. However, it cannot accommodate for geometrical mismatch without creases, especially as the dressing surface area increases. Since the dressing is stiffer and thicker, these creases are difficult to seal in an air-tight manner, due to its increased bending stiffness. Therefore, hydrocolloids are often only applicable to smaller wounds. Much effort is currently being taken to make them thinner, in order to increase their applicable surface area and accommodate more for contours, such as the Replicare Thin Hydrocolloid Dressing by Smith and Nephew. Hydrocolloids rely on their extremely sticky adhesive properties to account for increased skin adhesion and reduced air leaks. If they come in contact with wound exudate, the polymers in the hydrocolloid swell with water until saturation, forming a gel, which is held solid in its adhesive matrix structure.
In the SNaP® Wound Care System, the hydrocolloid dressings are connected to the tubing with a mechanical connector component, similar to the T.R.A.C. Pad, KCI. The SNaP® Wound Care System eliminates any potential air leaks from this mechanical connector by prefabricating it to the center of the entire dressing during manufacture. The prefabrication eliminates any potential air leaks at the tube-to-dressing interface due to user interface and geometrical mismatch, but it is not capable of being moved on the dressing surface. Therefore, it may need to be placed on an inconvenient area of the wound, such as a location that is uncomfortable for the patient. Additionally, the tube runs parallel to the plane of the drape; the direction of the tube along the plane of the drape is fixed. Since the dressings are not typically round, the tube path may be required to travel in an undesirable path, in order to cover the wound area with the preset shape of the drape.
For its vacuum source, the SNaP® Wound Care System uses a complex system, driven by constant force springs. Therefore, as the pump expands, mainly due to air leaks and potentially exudate removal, the pressure remains relatively constant for the length of the pressure application. This system is expensive and highly technical when compared to the non-electrical pumps at the low end of the medical pump spectrum (e.g., bladder pumps); however, it is the first commercial mechanical NPWT pump, which has been proven to be a potential NPWT pump design. Since air leaks into the dressing system remain highly probable, depending on wound location and caregiver experience, the successful application of the SNaP® Wound Care System is limited in practice.